In my opion you were short on rx battery !

LIon batterys cannot supply a large amount of current draw. That is the reason they arerun in 2S2P configuration totaling 5200Mah.{the most popular config} The 5200 X2 In redundant form is not needed for capicity BUT is needed to prevent voltage sag in hi amperage conditions.The Kingcat is loaded with a bunch of digital servos and if you were running digitals on the flaps compounds the problem. Digital servos on the flaps in the deployed position if they are not set up properly can draw 3-4 amps by themself.When the flaps are deployed the servo arm and the pushrod should be in a straight line or almost. You do not want theservo motor holding the flight load. A servo that hangs up can draw between 1-2 amps by itself. The way LION batterys are tested is to load the battery with a 1 amp load which will take a freshly charged battery from 8.4 volts down to 7.6 VDC. In my opion again your plane should have had at min. 2X 2S2P 5200 OR another battery chemistry such as A123 or Nicad. NiMi has the same problems as LiON and more problems will surface as Nicad is phased out if you do not move to the newed chemistry.

Jerry